The present account deals with the period up to around 1950. It includes some references to major papers in this period (by author of year of publication), but the original version of this review in
Astronomy & Astrophysics Reviews
must be consulted for the full details about these.

Introduction

The history of cometary astronomy is naturally divided into five major periods, the transitions being marked by important new insights. Before 1600, comets were essentially considered to be heavenly omens and were not yet clearly established as
celestial
(astronomical), rather than meteorological
phenomena in the terrestrial atmosphere. Then followed two centuries of mostly positional measurements with emphasis on the motions and the orbits, lasting until the early 19th century, when the era of cometary physics
was inaugurated, in particular by the passage of P/Halley in 1835. The next major step forward occurred in 1950 with the sudden emergence of the modern picture of comets as being essentially very old solar system objects
made of primordial ice and dust, generally in unstable orbits and intensively interacting with the solar electromagnetic and corpuscular radiation. Finally, the space missions to P/Giacobini-Zinner in 1985 and especially to P/Halley in 1986 provided the first in situ observations
of comets and dramatically widened our scientific horizon, but also posed many new questions which are yet to be answered.

Before 1950: The main events

The word
comet
, now used in all European languages, comes from Greek (
kometes
= `the hairy one'), but the earliest extant records of cometary observations date from around -1000 in China and probably from about the same time in Chaldea (on the territory of present-day Iraq). Ideas about the true nature of comets are available from the time of the rise of Hellenistic natural philosophy at about -550 when the Pythagoreans considered comets to be a kind of (wandering) planets that were seen rather infrequently and mostly near the horizon in the morning or evening sky. Aristotle in his
Meteorology
(ca. -330) relegated comets to the lowest, `sublunar' sphere in his system of spherical shells and described them as `dry and warm exhalations' in the upper atmosphere. There is no mention of comets in Ptolemy's Almagest
, presumably because they were not considered of celestial origin, but he described them in astrological terms in his
Tetrabiblos
. The Aristotelean view on comets was dogmatically upheld during the following millennium; the first doubts seem to have been expressed by Thomas Aquinas and also by Roger Bacon in his
Opus Tertium
from 1267, but like their predecessors they strongly believed comets to be evil omens.

Finally, Paolo Toscanelli observed P/Halley in 1456 and several other comets between 1433 and 1472 with improved accuracy, inaugurating the renaissance of European observational astronomy after the long period of dormancy. The decisive demonstration was delivered by Tycho Brahe (and confirmed by a few other observers, especially Michael Mдstlin), on the basis of extensive observations of the bright comet which first appeared in late 1577. He showed that the horizontal parallax of this comet was certainly smaller than 15 arcmin, corresponding to a distance in excess of 230 Earth radii, or four times the distance to the Moon. The question of how comets move arose as a natural consequence and in 1610, the amateur Sir William Lower proposed that they do so in very elongated ellipses, while Robert Hooke and Giovanni Borelli suggested that cometary orbits may be parabolic. Georg Dцrffel was the first to specifically state that the two bright comets seen in 1680 and 1681 are one and the same before and after its perihelion passage, and that it moved along a parabola with the Sun in the focal point. Isaac Newton in
Principia
(1687), applied his new theory of gravitation to show that the 1680 comet moved in an elliptical, albeit very nearly parabolic orbit and that it passed only about 0.0016 AU above the surface of the Sun. Edmond Halley (1705) computed the orbits of a dozen well-observed comets and demonstrated the periodical nature of the bright comet of 1682. `Halley's Comet', as it was from now on called, was telescopically recovered in December 1758 by Johann Palitzsch; this proved conclusively the validity of Newton's law of gravity out to the distance of the aphelion at 35 AU, more than three times the distance of Saturn, the outermost planet known at that time.

18th century cometary astronomy is characterized by the gradual development of improved methods for orbital computations and at the beginning of the 19th century, this had become a straightforward, if still somewhat arduous task, in particular when planetary perturbations were taken into account by means of iterative corrections. Some basic features of the orbital distribution of comets were established, e.g. the extremely broad range of orbital periods, over which the different objects are scattered. While some comets turned out to have orbits virtually indistinguishable from parabolas, others were confined to the inner solar system in the vicinity of Jupiter's orbit or inside of it. As time passed, a concentration of comets moving in similar orbits with fairly low inclinations and with aphelia close to Jupiter's orbit became more and more obvious; this concentration became known as the
Jupiter family
. It either called for a continuing ejection from Jupiter or for a mechanism of dynamical evolution, called `capture', whereby the comets would become concentrated into such orbits. It was realized that comets in general, and Jupiter family members in particular, suffer by far the largest orbital perturbations due to the action by Jupiter, and the restricted three-body problem (Sun-Jupiter-comet) therefore offered an interesting approximation for the study of their dynamical behaviour.

After Halley, Johann F. Encke was the second to successfully predict the return of a comet (in 1822) which as a consequence now carries his name. It turned out to have, and still has, the shortest period of all known comets, 3.3 years, and it was soon found to arrive systematically about 0.1 days earlier at perihelion than predicted, even when taking all planetary perturbations into account. Inspired by his observations of an asymmetric distribution of luminous matter in the head of P/Halley in 1835, Friedrich W. Bessel interpreted this as a Sun-oriented asymmetric outflow and suggested that a
non-gravitational effect
might arise due to the rocket-type impulse imparted by such an outflow. As a consequence, such perihelion shifts as observed for P/Encke might arise.

During the next decades, progressively more sophisticated instrumentation became available and the road was opened for a more physical approach to the study of comets. Comet tails were explained by Heinrich W.M. Olbers (1812) and Bessel (1836) by assuming that they were made of solid particles on which was acting a repulsive force directed anti-sunward. The close connection between comets and meteors was demonstrated by Giovanni Schiaparelli (1866, 1867) who found that the orbits of the Perseid and the Leonid meteor streams coincide with those of comets P/Swift-Tuttle (1862 III) and P/Tempel-Tuttle (1866 I), respectively. In 1835, P/Halley became the first comet in which detailed structures were extensively observed, in particular by John Herschel, Bessel and Friedrich G.W. Struve, who described jets, cones and streamers, cf. the Atlas by Donn et al. (1986). This led Bessel (1836) to postulate the ejection of material in the direction of the Sun which was then somehow forced back in the opposite direction by an unknown repulsive force. Feodor A. Bredikhin (quoted by Jaegermann, 1903) further developed this interpretation into the
Bessel-Bredikhin
' mechanical model which remained in use until the late 1950's. Sir Arthur Eddington (1910), introduced the
fountain model
of particle ejection in which the parabolas represent the outer envelopes of particle trajectories emitted from the sunlit hemisphere of the nucleus or surfaces of high density of matter. One of the repulsive forces acting on the dust was identified by Svante Arrhenius (1900) as the radiation pressure
by sunlight. The corresponding theory was further developed by Karl Schwarzschild (1901) and extended to molecules by Peter Debye (1909).

The first spectroscopic observations of comets were made by Giovanni Donati (1864) and by Sir William Huggins (1868) who visually compared the spectrum of comet Winnecke (1868 II) with flame spectra and found that the bands seen in the comet and in the flame, now known as the `carbon' or `Swan bands', were similar. Subsequent observations showed that these bands were present in all comet spectra and that carbon was therefore an important constituent of comets. Spectroscopy soon became the standard technique for studying the light of comets and new emissions were discovered at an increasing rate; Baldet (1926) published a detailed description of the spectra of about 40 comets, obtained since 1864, together with a complete bibliography of all comets observed until that time by spectroscopy. Schwarzschild and Kron (1911) studied the intensity distribution in P/Halley's straight tail during the 1910 passage and suggested that the emission could be explained by the effect of absorption of solar light, followed by re-emission, i.e.
fluorescence
. Polydor Swings (1941) solved the long-standing problem of why the violet CN bands (3875 A) in cometary spectra did not resemble CN laboratory spectra and varied in appearance: because of the crowding of absorption lines in the solar spectrum, the intensity at the exciting wavelengths critically depends on the doppler shift caused by the comet's motion relative to the Sun and so determines the strength of the fluorescence emission lines in the comet's spectrum; this is now known as the
Swings effect
.

1950 - 1951: Two crucial years

A major revolution in cometary science took place in 1950-51, with the formulation within a short time span of three fundamental ideas: 1) the
icy conglomerate
(`dirty snowball') model of the cometary nucleus
by Fred Whipple (1950), 2) the identification from kinematic studies of the existence of a distant reservoir of comets, now known as the Oort cloud
, by Jan Hendrik Oort (1950), and 3) the explanation of the motions in cometary plasma tails as due to interaction with the
solar wind
by Ludwig Biermann (1951). Interestingly, none of these ideas resulted directly from new observational evidence, and important parts of them had been proposed earlier, but it was the first time that the known facts were effectively combined to reveal the new picture.

The icy conglomerate nucleus

Karl Wurm, in a series of enlightening papers published between 1932 and 1939, suggested that, because the observed cometary radicals and ions are not chemically stable, these species must be created by pure
photochemistry
of more stable molecules residing inside the nucleus, cf. for instance the reviews by Wurm (1943) and Swings (1943). In the 1940's, Swings contributed significantly to the development of ideas along Wurm's line of thinking and his key role appears to have been overlooked in the later literature. The presence of CO, C 2 N 2 , CH 4 , CO 2 , N 2 and NH 3 was invoked on the basis that CO + , CN, CH, CO 2 + , N 2 + and NH were identified in comet spectra, respectively. Swings proposed many possible and reasonable candidates as parent molecules, among others CH 4 since CH 2 was held responsible for the emission recorded in the 4000-4100 A interval, as well as H 2 O, following the discovery of the OH 3090 A ultraviolet emission in 1941 by Swings, and despite the fact that the low vapor pressure of water was considered a serious problem when explaining the observed presence of the OH emission far from the Sun. In 1948, he came very close to actually proposing an icy model
for the nucleus by suggesting that the mentioned molecules could exist in the solid state in the nucleus.

Swings (1942) also suggested that molecules similar to those found in meteorites were possibly stored in the nucleus by occlusion. This idea was quantitatively (and most probably, independently) explored by Boris Yu. Levin (1943), who developed the
desorption
theory of outgassing from the surface of meteoritic material to demonstrate that his sand bank
model for the nucleus had a solid basis. However, although the average desorption heat, about 6000 cal/mole, as deduced empirically from the observed brightness/heliocentric distance relation, was in agreement with the laboratory values for the cometary molecules mentioned above, the amount of material that could be desorbed from a sand bank with an expected cometary mass fell far short of explaining the persistence of comae over several months at single passages, or indeed, the survival of comets like P/Halley or P/Encke for many apparitions.

Since the mid-19th century, a great deal of research had concentrated on understanding the nature of the central source of gas and dust in comets. Transits of comets across the solar disk had never shown any dark silhouette, proving the absence of any extended, optically thick object. Seeing-limited observations of comets passing near the Earth showed a central, unresolved light source of dimensional upper limits in the 10-100 km range (Nicolaus B. Richter, 1963). Upper limits to cometary masses had been estimated for instance from the absence of evidence for mutual gravitational attraction of the components of P/Biela in 1846 or of any influence on the Earth's orbit at very close passages like that of P/Lexell in 1770; in the end, masses in the 10 12 - 10 17 kg range were estimated (Whipple 1961). Comets were obviously small and light bodies, possibly even without a solid nucleus at the center. At the end of the 1940's, the nature of the nucleus was still a subject of much speculation and no consensus had been reached. In an attempt to put together all known facts about the cometary nucleus, and with particular attention to the long-standing problem of explaining the non-gravitational perihelion shifts, Whipple (1950, 1951) laid the foundations for the model of an
icy conglomerate, solid nucleus
. Building on the idea dating back to Pierre S. de Laplace (1813) and Bessel (1836), Whipple described the nucleus as a mixture of
ices
from which the gases in the coma are produced by sublimation in increasing quantities as the comet approaches the Sun and the nucleus surface temperature rises, and meteoritic dust
that is released from the nucleus when these ices evaporate. This model had the virtue of explaining at once several observed features: 1) the large gas production rates, for which the desorption model was totally inadequate, 2) the observed jet-like structures in the coma and the erratic activity, impossible to produce if the nucleus were a cloud of particles, 3) the observed non-gravitational forces by means of momentum transfer by the outflow of gas from the nucleus, the net effect on the orbital motion being dependent, among others, on the sense of the nuclear spin and the direction of the spin axis, 4) the fact that most comets which pass extremely close to the Sun, e.g. the Kreutz sungrazing group, apparently may survive such approaches intact and with little change after perihelion, and 5) the fact that comets are the sources of meteor streams. Items 2-4 gave particularly strong arguments for a solid nucleus rather than a sand bank structure.

The Whipple model quickly won general acceptance and was gradually refined during the following decades. It had, however, some shortcomings; the main one was pinpointed by Whipple himself as being the large difference between the latent heats of vaporization of the various ices. As a consequence the highly volatile material should be rapidly removed from the surface layer of the nucleus long before perihelion, in contradiction to the observation of radicals and ions like CH and CH + near the Sun. This objection was tentatively removed when Armand Delsemme and Swings (1952) noticed that almost all parent molecules (except NH 3 ) required to explain the observed radicals and ions in comets could co-exist in the nucleus in the form of solid
clathrate hydrates
. In this way, the highly volatile material does not disappear too rapidly and is also freed together with less volatile molecules; this explains why the spectrum remains more or less similar throughout the comet apparition.

The Oort Cloud

Many orbital studies of individual comets with particular attention to the influence of planetary perturbations were carried out at several observatories during the first decades of the 20th century. They were naturally followed by statistical considerations about the distribution and dynamical origin of comets, including the question of whether or not some comets have `original' hyperbolic orbits (reciprocal semi-major axis 1/a orig < 0) and are therefore of interstellar origin. The work at the Copenhagen Observatory by Elis Strцmgren (beginning in about 1910) and his associates is typical of such studies and showed the absence of originally hyperbolic orbits, all observed orbits of this type having been caused by planetary perturbations. Sinding (1948) produced a list with the values of 1/a orig for 21 long-period comets which together with the work by van Woerkom (1948) formed the basis for Oort's famous paper (1950) on the existence of a cometary reservoir in the outer reaches of the solar system. The idea of a cloud of distant
hypothetical
comets, stable against stellar perturbations, and its necessity in case many observed comets would have 1/a orig 10000 AU, had been expressed earlier by Ernst J. Цpik (1932).

Based on van Woerkom's (1948) theory of the orbital diffusion caused by planetary perturbations, Oort found that the number of comets with very small values of 1/a orig is much larger than one would expect, when comparing with the neighbouring, long-period elliptical orbits. This suggested that many of the comets become unobservable after their first passage through the inner solar system. In a subsequent study, Oort and Schmidt (1951) distinguished between
`new comets'
(those coming directly from the Oort cloud, making their first visit near the Sun) and `old comets'
(those returning on elliptic orbits). The former appeared to be dustier and brighten more slowly than the latter. These tentative conclusions have later been revisited and modified, and the role of stellar perturbations in providing new comets has been reconsidered. However, the basic concept of the Oort cloud as an outer halo of the solar system has been substantiated by later studies, based on improved samples of cometary orbits.

The solar wind

The tails of comets have been the objects of many investigations. It is exactly these appendices that make comets so impressive to the layman, and astronomers of all times have been struck by the fact that the tails may vary so dramatically from one object to another. In the early 20th century, perhaps the strangest characteristic was the enormous repulsive force found to act on straight comet tails in the anti-solar direction.

Already in 1859, Richard Carrington (1859) suspected a physical connection between the major solar flare observed in the morning of Sept. 1, and enhanced magnetic activity on the Earth some hours thereafter. Ideas about the possible existence of a stream of particles from the Sun, perhaps electrically charged, emerged towards the end of the 19th century, in particular to explain the excitation of molecules and ions observed in cometary comae. It was also found that cometary ion tails (formerly Type I) develop closer to the Sun than dust tails (formerly Type II). However, it was only 50 years later that Cuno Hoffmeister (1943) provided the crucial observations of a gas tail
aberration
of about 6 o , i.e. the angle between the observed tail and the anti-solar direction. This was correctly interpreted by Biermann (1951) in terms of interaction between the cometary ions in the tail and the solar wind
, a stream of electrically charged particles from the Sun with velocities of several hundred km/sec. His derived plasma densities were unrealistically high, since electrons were thought to accelerate the cometary ions, but Hannes Alfven (1957) settled this problem by introducing the notion of an
interplanetary magnetic field
which is carried along with the solar wind. Its existence was soon thereafter confirmed by experiments onboard some of the first spacecraft launched after the space age opened in late 1957 ( Lunik I
and II
,
Explorer X
,
Mariner II
, etc.). Still, for quite some time, cometary ion tails were the only well-distributed solar wind probes in interplanetary space and they remain so outside the ecliptic. Another important result of Alfven's study is that an ion tail must be considered as part of the comet since it is magnetically connected to the cometary head.